The present invention relates to a configuration of and a method of assembling a regenerative cooled tubular rocket engine combustion chamber and nozzle.
A rocket engine combustion chamber contains the combustion of pressurized fuel and oxidizer and the smooth acceleration of the combustion products to produce thrust. Referring now to FIG. 1, the oxidizer and fuel are introduced under pressure through an injector 10 attached to the top of the chamber 12. The combustion products under pressure advance to a de Laval nozzle 14 where the internal profile converges to a throat 16. Here, the expansion of the combustion products achieves sonic velocity. The convergent throat section is immediately followed by a divergent section 18. The combustion products are then further accelerated to many times the speed of sound depending on the profile of the divergent section, the oxidizer and fuel combination, the pressure of the combustion products and the external pressure. The acceleration of gases creates thrust for the rocket engine.
Regenerative cooled combustion chambers take part of the flow of cryogenic liquid propellant, usually fuel, to cool the walls of the combustion chamber. The coolant flows along the outside of the chamber through passages or tubes. The coolant recycles the waste heat to increase energy in the coolant. This increased energy in the coolant improves the efficiency of the cycle. Regenerative cooled combustion chambers for rocket engines typically fall into three categories: milled channel; platelet; and tubular construction.
In a milled channel construction, grooves of varying cross section are cut into the exterior of a liner, which assumes the shape of a de Laval nozzle. A jacket is then built up over the open channels or a cylindrical piece is slid on and vacuum compression brazed to the liner.
A platelet construction is similar to a milled channel but divides the length of the liner into many smaller sections, which are then bonded together. A multiple piece jacket is then welded together over the liner and vacuum compression brazed together.
A tubular construction combustion chamber can be manufactured in two ways depending on its size. If the chamber is large enough in diameter to allow access, the tubes and braze material can be laid directly into a single piece jacket and furnace brazed.
Assembly of smaller chambers starts by stacking forward tubes on a mandrel in the shape of a de Laval nozzle. The tubes can be laid straight along the length of the mandrel or can be spiral wrapped around the mandrel. The tube ends are inserted into an inlet and exit manifold. Braze wire, paste and foil is inserted into all the cavities between the tubes. The tubes contain the pressurized propellants for cooling the chamber walls and picking up waste heat to use in the cycle. A multiple piece jacket is then added to the outside of the tubes. The jacket segments are then welded together or overlapping strips are added between the jacket segments. The jackets and tubes are then furnace brazed together.
The tubular construction chamber yields the lightest and most efficient chamber due to the larger heat transfer area and lower stressed tube cross sections. The tubular construction chamber integrity depends on the quality of construction of the multiple piece jacket and braze coverage for all joints between the tubes, jacket segments, and manifolds.
The majority of the heat transfer between the combustion products and the coolant in the combustion chamber occurs from the injector face down past the throat to a few inches beyond the throat. It is here that the advantages of the tubular construction chamber are best used to increase the efficiency of the system. The point downstream of the throat where heat transfer between the coolant and chamber drops off, is the best place to split the assembly into a combustion chamber (upstream) and a nozzle (downstream) for manufacturing and assembly purposes. The nozzle can be of any smooth wall construction to reduce costs.
Smooth wall nozzles can be any of four types: milled channel; platelet; ablative; and radiation. The first two have been described above. The ablative nozzle employs a coating on the chamber internal profile that releases a cooling gas as it is heated by the combustion products. A radiation nozzle is made from a material that can take the heat input from the combustion products and give positive structural margin while only relying on radiation cooling.
Current engines use a regenerative cooled milled channel chamber and nozzle, a regenerative cooled milled channel chamber with a regenerative cooled tubular nozzle, or a combination all tubular construction chamber and nozzle.
The difficulty with incorporating a regeneratively cooled tubular chamber with a smooth wall nozzle lies in the transition zone between a tubular chamber wall profile and a smooth wall profile. Due to the very thin boundary layer of cooler gas flowing along the chamber profile, any sudden disturbance in the profile can lead to excessive temperatures locally in the wall and the generation of shocks. Higher temperatures reduce the material capability to withstand the internal coolant pressure loading and could lead to sudden failure. Shocks can lead to local pressure loading which can overload chamber wall material.